Download Essentials of Geology Earthquakes and Earth`s

Essentials of Geology
David Sallee
Earthquakes and
Earth’s Interior
Chapter 14
An aerial view of the San Andreas fault
in the Carrizo Plain, Central California
What is the Elastic Rebound
Theory?
What are Earthquakes?
„
„
„
The shaking or trembling caused by the
sudden release of energy
Usually associated with faulting or
breaking of rocks
Continuing adjustment
of position results in
aftershocks
„
Explains how
energy is stored
in rocks
• Rocks bend until
the strength of the
rock is exceeded
• Rupture occurs
and the rocks
quickly rebound to
an undeformed
shape
• Energy is released
in waves that
radiate outward
from the fault
What is Seismology?
What is Seismology?
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The Focus and Epicenter of an
Earthquake
The point
within Earth
where faulting
begins is the
focus, or
hypocenter
The point
directly above
the focus on
the surface is
the epicenter
Seismographs
record earthquake
events
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At convergent boundaries,
boundaries,
focal depth increases
along a dipping seismic
zone called a Benioff zone
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Where Do Earthquakes Occur
and How Often?
„
Where do Earthquakes Occur
and How Often?
~80% of all
earthquakes occur in
the circumcircum-Pacific belt
• most of these result from
convergent margin
activity
• ~15% occur in the
MediterraneanMediterranean-Asiatic
belt
• remaining 5% occur in
the interiors of plates and
on spreading ridge
centers
• more than 150,000
quakes strong enough to
be felt are recorded each
year
Damage in Oakland, CA, 1989
What are Seismic Waves?
What are Seismic Waves?
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Response of
material to the
arrival of
energy fronts
released by
rupture
Two types:
Body waves
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Surface waves
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„
Body waves
• P or primary waves
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„
„
fastest waves
travel through solids,
liquids, or gases
compressional wave,
material movement is
in the same direction
as wave movement
• S or secondary waves
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„
• P and S
„
slower than P waves
travel through solids
only
shear waves - move
material perpendicular
to wave movement
• R and L
How is an Earthquake’s
Epicenter Located?
What are Seismic Waves?
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„
Surface Waves
• Travel just below or along the ground’s surface
• Slower than body waves; rolling and sideside-toto-side
movement
• Especially damaging to buildings
Seismic wave
behavior
• P waves arrive first,
then S waves, then L
and R
• Average speeds for all
these waves is known
• After an earthquake,
the difference in
arrival times at a
seismograph station
can be used to
calculate the distance
from the seismograph
to the epicenter.
2
How is an Earthquake’s
Epicenter Located?
„
TimeTime-distance
graph showing the
average travel
times for PP- and SSwaves. The farther
away a seismograph
is from the focus of
an earthquake, the
longer the interval
between the
arrivals of the PPand SS- waves
How is the Size and Strength
of an Earthquake Measured?
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Modified Mercalli Intensity Map
• 1994 Northridge, CA earthquake, magnitude 6.7
Intensity
How is an Earthquake’s
Epicenter Located?
„
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„
How is the Size and Strength of
an Earthquake Measured?
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Ground
Shaking
• amplitude,
duration,
and
damage
increases
in poorly
consolidat
ed rocks
Magnitude
• Richter scale measures
total amount of energy
released by an
earthquake;
independent of
intensity
• Amplitude of the
largest wave produced
by an event is
corrected for distance
and assigned a value
on an openopen-ended
logarithmic scale
• subjective
measure of
the kind of
damage
done and
people’s
reactions to
it
• isoseismal
lines
identify
areas of
equal
intensity
What are the Destructive
Effects of Earthquakes?
What are the Destructive
Effects of Earthquakes?
„
Three seismograph
stations are needed
to locate the
epicenter of an
earthquake
A circle where the
radius equals the
distance to the
epicenter is drawn
The intersection of
the circles locates
the epicenter
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Building
collapse
Fire
Tsunami
Ground
failure
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Can Earthquakes be Predicted?
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Can Earthquakes be
Predicted?
Earthquake Precursors
• changes in elevation or tilting of land surface,
fluctuations in groundwater levels, magnetic field,
electrical resistance of the ground
• seismic dilatancy model
• seismic gaps
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• include laboratory and field studies of rocks before, during, and
and
after earthquakes
• monitor activity along major faults
• produce risk assessments
Can Earthquakes be
Controlled?
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What is Earth’s Interior Like?
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A profile showing
seismic velocities
versus depth
• several discontinuities
indicate changes in
Earth materials or their
properties
• discontinuities are the
basis for subdividing
Earth’s interior into
concentric layers
Crust, mantle, core
• behavior and travel
times of P and S
waves helps define
interior structure
• velocity of waves is
dependent on the
density and elasticity
of material they
travel through
• waves are bent
(refracted) or
bounced (reflected)
as they pass through
different materials in
Earth
Graph showing the
relationship between
the amount of waste
injected into wells
per month and the
average number of
Denver earthquakes
per month
Some have
suggested that
pumping fluids into
seismic gaps will
cause small
earthquakes while
preventing large
ones
What is Earth’s Interior Like?
„
Earthquake Prediction Programs
The Core
„
Density and
Composition of the Core
• behavior of P and S
waves indicates a solid
inner and liquid outer
core
• inner core is iron/nickel,
rotates more rapidly than
outer core
• outer core is iron mixed
with sulfur, density of 9.9
to 12.3 gm/cm3
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The Mantle
The Mantle
Structure and
Composition of the
Mantle
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• Seismic wave velocities
generally increase with
depth, but several other
discontinuities exist
• The lowlow-velocity zone is
inferred to represent
zones of partial melting in
the asthenosphere
• Composition believed to
be that of the igneous
rock peridotite
Discontinuities
• Sharp velocity increase in wave travel times at a
depth of about 30km - called the Moho
• The Moho separates the crust from the mantle
Seismic Tomography
„
ThreeThree-dimensional
models of Earth’s
interior
• More complete
analysis of seismic
waves indicated hot
and cold areas
within the mantle
• Hot areas underlie
spreading centers
and volcanic areas
• Cold areas underlie
the older interior
portions of the large
continents
The Crust
„
Continental crust
• Overall composition similar to granite
• Low density (2(2-3gm/cm²)
• Averages 35km thick, more under mountain
ranges
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Oceanic crust
• Gabbro/basalt composition
• Higher density (3gm/cm²)
• Between 5 and 10 km thick
Earth’s Internal Heat
„
Geothermal gradient
• Temperature increases with depth
• Averages 25°C/km
• Gradient is higher in areas of active or
recently active volcanism
• Most heat is generated by radioactive decay
• Maximum temperature at the center of the
core is estimated to be about 6,500°C
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